Primary Rat Hepatocytes Express Cyclin D1 Messenger RNA During Their Growth Cycle and During Mitogenic Transitions Induced by Transforming Growth Factor-α

Cyclin D1 messenger RNA expression was investigated in differentiated proliferation-competent primary cultures of adult rat hepatocytes. Periodic expression was observed during the 12-day growth cycle (≍two population doublings). Upon reaching a quiescent G0-state, 4-kb cyclin D1 mRNA levels were undetectable. When G0 cultures were shifted into defined media without or with 0.2-0.8 nM TGF-α, conditions that reinitiate full proliferative transitions synchronously, cyclin D1 mRNA levels were elevated 1.2-4.6-fold, respectively. These findings support the hypothesis that hepatic mitogens stimulate periodic cyclin mRNA expression directly.